1. Field of the Invention
The present invention relates to an image pickup system. In particular, the present invention relates to an image pickup system having an infrared cut function.
2. Description of Related Art
In optical systems for a digital still camera and a digital video camera, light-receiving elements such as CCD or CMOS are used. These light-receiving elements have a high sensitivity not only in a visible region but also in an infrared region. Thus, in order to provide a light-receiving characteristic close to the visibility, it is required that light having a wavelength λ of 800 to 1200 nm is cut. Conventionally, an infrared cut filter in which dozens of high refractive index materials and low refractive index materials were alternately layered on a glass substrate, has been used.
However, an optical element in which the infrared cut filter is formed on a glass substrate, is expensive. Further, because the glass substrate is thick, it is difficult to fully satisfy the recent requirements for a smaller and thinner optical system. A technique in which a smaller and thinner optical system is provided by forming an infrared cut filter on one surface of the outermost lens in an image pickup system lens, is known (for example, Japanese Patent Unexamined Publication No. 10-10423).
However, in case that an image pickup system lens is made of plastic, when a laminated film composed of dozens of layers for cutting light having wavelength λ of 800 to 1200 nm, is formed, there was a problem of the significant technical difficulty because of a risk of surface deformation or the like due to crack or stress. Furthermore, when a glass lens as an image pickup system lens is coated with dozens of infrared cut filters, there was a problem that the cost of the above coating is the same as that of an infrared cut coating of a glass substrate.